1. Field of the Invention
The present invention relates to a vibrator used for an angular rate sensor used for detecting a turning angular rate in a turning system and a vibratory gyroscope using the same vibrator, and particularly to a vibrator using a piezoelectric member and a vibratory gyroscope using the same vibrator.
2. Description of the Related Art
Up to now, as an angular rate sensor used for detecting a turning angular rate in a turning system, a vibratory gyroscope using a piezoelectric member has been used for detecting position of an aircraft, a ship, a space satellite, or the like. Recently, it is used in a car-navigation system, a movement detecting mechanism of a VTR or a still camera, and the like in the field of public livelihood.
Such a vibratory gyroscope utilizes that when an angular speed is applied to a vibrating object, a Coriolis force is generated in the direction perpendicular to the vibratory direction. Its mechanism is analyzed by using a dynamic model (for example, "Handbook of Elastic Wave Device Technologies" (Danseiha-Sosi Gijutsu Handbook) issued by Ohm, Inc., pp.491 to 497). Various kinds of piezoelectric vibratory gyroscopes have been proposed up to now. For examples, a Sperry tuning-fork gyroscope, a Watson tuning-fork gyroscope, a regular-triangle prism-shaped tuning-piece gyroscope, a cylindrical tuning-piece gyroscope, and the like are known as a piezoelectric vibratory gyroscope.
The inventors are studying various applications of vibratory gyroscopes, and have studied using a vibratory gyroscope as a turning rate sensor used in a car control method of an automobile body turning rate feedback system, for example. Such a system detects the direction of a steering wheel itself by a turning angle of the steering wheel. At the same time as this, the system detects a turning rate of the actually turning car body by means of a vibratory gyroscope. The system finds a difference between the direction of the steering wheel and the actual body turning rate by comparing them with each other, and attains a stable body control by compensating a wheel torque and a steering angle on the basis of this difference.
However, any example of the above-mentioned former piezoelectric vibratory gyroscopes can detect a turning angular rate only by arranging a vibrator in parallel with the axis of turning (what is called a vertical arrangement). The turning axis of a turning system to be measured is usually perpendicular to the gyroscope mounting part. Accordingly, in mounting such a piezoelectric vibratory gyroscope it has been impossible to shorten the piezoelectric vibratory gyroscope in height, namely, to reduce the piezoelectric vibratory gyroscope in size in the direction of the turning axis.
In recent years, a piezoelectric vibratory gyroscope capable of detecting a turning angular rate even when arranging a vibrator perpendicularly to the turning axis (what is called a horizontal arrangement) has been proposed in a Japanese laid-open publication Tokkaihei No.8-128833. In this example, as shown as an example in FIG. 1, a vibrator extends in the directions X and Y, namely, extends perpendicularly to the turning axis Z. Each of three elastic members 51a, 51b and 51c is provided with a weight 53 at one end of it. The elastic members 51a, 51b and 51c are vibrated by piezoelectric devices 54 and 55 in the X-Y plane in phase inverse to one another. A Coriolis force in the Y direction generated by a turning angular rate .omega. around the Z axis is applied to the center of gravity of the weight 53. Since the plane of the elastic members 51a, 51b and 51c and the center of gravity of the weight 53 are slightly distant in the Z direction from each other, the ends of the elastic members 51a, 51b and 51c are bent reversely to one another in the Z direction by the Coriolis forces each of which is applied to the center of gravity of the weight 53. A turning angular rate .omega. around the Z axis is obtained by detecting this bending vibration by means of piezoelectric devices 56 and 57.
And up to now, various compositions have been known as a vibratory gyroscope using a vibrator which is composed of plural arms and a base part joining the plural arms, gives a drive vibration in a specified plane to each of the arms, and obtains a turning angular rate on the basis of a detection vibration which is perpendicular to this drive vibration and corresponds to the applied turning angular rate. For example, a Japanese laid-open publication Tokkaihei No. 7-83671 has disclosed a vibratory gyroscope using a tuning-fork vibrator made by joining three arms composed of a middle drive arm and two detection arms at both sides of the middle drive arm in one body at the base part. FIG. 2 shows a composition example of such a former vibratory gyroscope. In the example shown in FIG. 2, a vibrator 102 forming a vibratory gyroscope is composed of three arms which are composed of a middle drive arm 104 and two detection arms 103 and 105 arranged at both sides of it nearly in parallel with it, and a base 106 at which the drive arm 104 and the detection arms 103 and 105 are joined in one body with one another.
In the above-mentioned tuning-fork vibrator 102, the drive arm 104 is vibrated in the X-Z plane by an unillustrated driving means provided on the drive arm. And the left and right detection arms 103 and 105 are resonated in the same X-Z plane. When a turning angular rate .omega. acts around the axis Z of symmetry of the tuning-fork vibrator 102, a Coriolis force f acts on each of the detection arms 103 and 105. Since the detection arms 103 and 105 are vibrating in the X-Z plane, vibration in the Y-Z plane is induced in the detection arms 103 and 105. A turning angular rate is measured by detecting this vibration by means of an unillustrated detecting means provided on each of the detection arms 103 and 105.